Mobile Circuit Switched (CS) services based on Global System for Mobile communications (GSM) and Wideband Code Division Multiple Access (WCDMA) radio access allow obtaining telecommunication services with a single subscription in almost all countries of the world. Also today, the number of CS subscribers is still growing rapidly, boosted by the roll out of mobile CS services in dense population countries such as India and China. In addition the classical Mobile Switching Center (MSC) architecture is changed into a so-called softswitch solution which allows using packet transport infrastructure for mobile CS services.
The third generation partnership project (3GPP) has completed the work item “Evolved UTRA and UTRAN” that defines a Long-Term Evolution (LTE) concept to assure competitiveness of 3GPP-based access technology. It was preceded by an extensive evaluation phase of possible features and techniques in the Radio Access Network (RAN) workgroups that concluded that the agreed system concepts can meet most of the requirements and no significant issue was identified in terms of feasibility.
In parallel to the RAN standardization 3GPP has also driven a System Architecture Evolution (SAE) work item to develop an evolved core network. The SAE core network is made up of core nodes, which are further, according to an Ericsson proposal split, into Control Plane (Mobility Management Entity, MME) and User Plane Gateway (Serving Gateway, S-GW and PDN Gateway, PDN GW) nodes. The resulting network architecture is shown in FIG. 1. Thus, FIG. 1 depicts nodes and interfaces in an LTE radio network and also a terminal (UE) connected thereto.
Common to both LTE and SAE is that only a Packet Switched (PS) domain has been specified, i.e. all services are to be supported via this domain. GSM and WCDMA however provide both PS and CS access simultaneously.
A solution called “CS Fallback” has been standardized and the stage 2 solution is defined in 3GPP Technical Specification TS 23.272 and FIG. 2 shows the “CS Fallback” architecture. In FIG. 2 the nodes and related interface between the nodes of a cellular radio network employing a CS Fallback solution is depicted. The nodes are typically implemented using hardware and software adapted to perform the different tasks associated with a respective node and also with communication means for digitally connecting with other nodes using the depicted interfaces.
The main principle of “CS Fallback” is that the terminal, i.e. the mobile station (MS) or User Equipment (UE) is performing normal SAE Mobility Management (MM) procedures towards the MME while camping on LTE radio access. The MME registers the terminal in the MSC Server (MSC-S) for CS based services using the SGs-interface. When a page for CS services is received in the MSC-S it is forwarded to the terminal via the MME (using the SGs-interface) and then the terminal performs fallback to GSM EDGE Radio Access Network (GERAN) or Universal Terrestrial Radio Access Network (UTRAN) and responds to paging via the selected RAN. Similar behavior applies for CS services originating in the terminal (mobile station). When these are triggered and the terminal is camping on LTE access (evolved UTRAN, E-UTRAN), the terminal performs fallback to GERAN or UTRAN and triggers initiation of the CS service there. Different solutions have been discussed for the “fallback” mechanism, such as PS Handover and inter-Radio Access Technology (RAT) cell change order (possibly with NACC, Network Assisted Cell Change)
FIG. 3 shows the case of a Mobile Station initiating a call in Active Mode and when PS Handover (HO) is supported. It is further described in section 6.2 of 3GPP TS 23.272, and one particular step, step 3, is further described in 3GPP TS 23.401 and depicted in FIG. 4.
Once the terminal (MS/UE) is done with the CS service in CS domain, it may return back to E-UTRAN using existing mechanisms or may be kept in 2G or 3G coverage, i.e. provided radio access via GERAN or UTRAN.
In idle mode, a mobile station normally performs cell reselection using information received in system information from the network. The network may however decide to change the cell reselection behavior for individual terminals. For example, the system information might be set to favor LTE before WCDMA and GSM but the network might detect that a particular user is better served in GSM. In such a case, the network can change the cell reselection parameters for that terminal so it will stay in GSM while in idle mode. It is however typically important and beneficial to keep a preferred distribution of UE:s in idle mode between GSM/WCDMA systems and LTE.
When a CS connection to an E-UTRAN capable MS/UE being connected to a 2G or 3G network is about to be terminated/released, the Base Station Controller/Radio Network Controller (BSC/RNC) may decide whether:                To send the MS/UE to E-UTRAN using explicit signaling, also known as release in GERAN or Radio resource Control (RRC) Connection Release with Redirect in UTRAN or by including the “Cell selection indicator after release of TCH and SDCCH” in CHANNEL RELEASE in GERAN.        To keep the UE in the current RAT (for example GSM or WCDMA)        
There is a desire to improve the performance in cellular radio networks. In particular there is a desire to improve performance in deployment scenarios where many radio access technologies such as GERAN/UTRAN and E-UTRAN are deployed in the same area and where fallback to CS is used.